In common methods for producing fluids from a well drilled into a petroleum-bearing subsurface formation, a string of steel production tubing is positioned in the wellbore and extends from the subsurface production zone up to a wellhead at the surface. A downhole pump is disposed within the production tubing in the production zone to raise well fluids (e.g., oil, gas, formation water) to the surface, by reciprocating vertical movement of a travelling valve incorporated into the pump. The travelling valve is reciprocated by a pump rod string (or “sucker rod” string, or simply “pump rod”) extending upward within the production tubing to the wellhead where it connects to a “polished rod” extending upward through a wellhead tee and stuffing box to connect to a pumping unit. This type of pump is commonly referred to as a positive-displacement pump or “sucker rod pump”.
Various types of well pumping units have been developed for operating positive-displacement downhole pumps, with the most common type being a “pump jack” comprising a walking beam mechanism that reciprocates the sucker rod string connected to the downhole pump, by means of a drive train comprising an electric motor or internal combustion engine, a gear reduction mechanism, and a braking system. The walking beam style of pumping unit is large, heavy, and expensive to build. If the single cable connecting the free end of the walking beam to the polished rod at the upper end of the pump rod string should break, the pump rod string will fall uncontrollably, damaging the well head and possibly losing the entire rod string into the well hole, entailing costly repairs and creating a safety hazard.
It is known to modify a walking beam pump jack to incorporate a counterweight system in order to reduce the total weight that needs to be lifted by the pump jack's drive system. During the upstroke of a downhole pump, the pump jack must lift the total weight of the sucker rod string plus the column of well fluids above the downhole pump's travelling valve. For example, for a rod string that weighs 15,000 pounds (including the travelling valve) and is required to lift a fluid column weighing 10,000 pounds, the pump jack will need to lift a total of 25,000 pounds on each upstroke. At the top of each upstroke, the pump jack's drive system must be disengaged to initiate a downstroke allowing the travelling valve to fall to the bottom of the well. On the downstroke, the 15,000-pound rod string is essentially in a controlled freefall through the liquid in the production tubing. Accordingly, the pump jack apparatus needs to incorporate a robust braking system to regulate the speed of the downstroke.
In a counterweighted pump jack system, the counterweight ideally corresponds to the weight of the rod string plus half the weight of the fluid column to be raised. In the example above, the counterweight would ideally weigh 20,000 pounds (i.e., 15,000 pounds plus ½ of 10,000 pounds), such that the net required lifting force on the pump's upstroke would be only 5,000 pounds. At the top of the upstroke, there would be a net downward force of 5,000 pounds acting on the counterweight—i.e., 20,000 pounds for the counterweight minus 15,000 pounds for the rod string (there being no fluid column load on the downstroke). Therefore, the pump jack's drive system requires a net lifting capacity of only 5,000 pounds—i.e., to lift the rod string and fluid column on the pump's upstroke, and to lift the counterweight on the pump's downstroke. This is in contrast to a non-counterweighted pump jack, which lifts only on the upstroke, but the required lifting capacity is dramatically reduced, as are the braking requirements.
Because a counterweighted pumping unit will typically need to lift on both the upstroke and the downstroke of the downhole pump, the unit's drive system must be reversible. The drive systems of most known pump jacks use conventional electric motors, which rotate in only one direction. Therefore, the use of such motors in counterweighted pumping units requires a reversing mechanism of some type. A suitable control system is provided to alternate the pump stroke direction at the end of each upstroke or downstroke.
One example of a prior art counterweighted pumping unit driven by an electric motor is the Rotaflex® unit manufactured by Weatherford® International Ltd., of Houston, Tex. The Rotaflex® unit has a vertical tower structure and an electric motor at the base of the tower. A gearbox is fitted to the motor's output shaft, and a drive sprocket is mounted to the gearbox. A continuous drive chain is trained around the drive sprocket and around an idler sprocket mounted in an upper region of the tower. A counterweight is connected to a selected link in the drive chain such that the counterweight will move vertically with the drive chain. A mechanical reversing mechanism is provided to alternate the travel direction of the drive chain and in turn the travel direction of the counterweight.
A discontinuous load belt is deployed over an idler roller mounted at the top of the tower, with one end of the load belt being connected to the counterweight and with the other end of the counterweight being connected to the polished rod of a sucker rod string associated with a wellhead. The rotational axis of the idler roller is transverse to the rotational axes of the drive chain sprockets, not parallel. By virtue of the connection of the counterweight to both the drive chain and also to the load belt, actuation of the electric motor causes the load belt to raise either the rod string or the counterweight, depending on the direction of travel of the drive chain (as controlled by the drive system's mechanical reversing mechanism).
The Rotaflex® unit thus provides the benefits of counterweighting in conjunction with a unidirectionally-rotating electric primary drive motor, but has the drawback of requiring complex mechanical apparatus in order to provide the necessary lifting capacity on both the upstroke and downstroke of the downhole pump being actuated by the unit. Particular examples of this mechanical complexity include the need for a gear reducer at the electric drive motor's output shaft (which rotates much faster than the drive sprocket), the specialized mechanical reversing mechanism, and the need for both a drive chain arrangement for reciprocating the counterweight plus a load belt arrangement for transferring lifting force to the rod string during the upstroke of the downhole pump.
U.S. Pat. No. 4,226,404 (Zens) discloses a counterweighted pumping unit that uses a reversible hydraulic motor actuated by a hydraulic pump. The hydraulic motor is directly coupled to a drum so as to rotate the drum about a horizontal axis. A pair of sheaves are provided, one on either side of the drum, with rotational axes generally parallel to the rotational axis of the drum. A first traction cable is fixed at one end to a first selected point on the perimeter of the drum and trained over a first one of the sheaves, with its other end being connected to a counterweight assembly. A second traction cable is fixed at one end to a second selected point on the perimeter of the drum and trained over the second sheave, with its other end being connected to a rod string associated with a wellhead. Rotation of the drum in a first direction will result in the rod string being raised and the counterweight being lowered; rotation of the drum in the opposite direction will result in the counterweight being raised and the rod string being lowered.
The illustrated embodiments of the Zens apparatus include one or more load cables trained over the sheaves, and connected at their opposite ends to the counterweight and to the rod string. The load cables do not engage the drum and therefore are not driven, but they serve to share the counterweight and rod string loads, preferably equally. To prevent uncontrolled lateral migration of the traction cables and load cables during operation of the apparatus, as well as interference between these cables, the perimeter surface of the drum is formed with a continuous helical groove for receiving and training the traction cables, and the perimeter surfaces of the sheaves are formed with parallel annular grooves for receiving and training the load cables.
Due to the helical groove in the drum, the lateral positions of the traction cables at and relative to the drum will shift (in a direction parallel to the drum's rotational axis) as the drum rotatingly oscillates from upstroke to downstroke. Because the lateral positions of the traction cables at the sheaves does not change during operation of the apparatus, the lateral shifting of the traction cables at the drum will cause a fleet angle to develop on each oscillation (i.e., the traction cables, unlike the load cables, will not remain perpendicular to the rotational axes of the drum and sheaves). This generally undesirable condition is overcome in the Zens apparatus by providing an ancillary mechanism for tilting the axis of the drum as necessary to compensate for the fleet angle(s) that would otherwise develop.
The Zens apparatus thus provides an example of a counterweighted pumping unit that avoids the need for gear reduction components and reversing mechanisms as in the Rotaflex® unit. However, it too has disadvantageous mechanical complexities, including the requirement for the large drum associated with the traction cables, the “highly desirable” load cables in addition to the traction cables, and the large sheaves also required for the traction cables and load cables. It is stated in the Zens patent that the size of the sheaves can be reduced by using additional cables; however, providing additional traction cables and load cables introduces additional complexity. A further drawback of the Zens apparatus is the inherent problem of fleet angles developing with respect to the traction cables, which is addressed by introducing further mechanical complexity in the form of a mechanism for constantly tilting the axis of the drum to keep the fleet angle equal to essentially zero.
For the foregoing reasons, there is a need for improved counterweighted pumping units having less mechanically complex drive systems than conventional counterweighted pumping units.